This application claims the priority of Korean Patent Application No. 2003-87994, filed on Dec. 5, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a compound semiconductor high frequency switch device that is a core element of a compound semiconductor switch Microwave Monolithic Integrated Circuit (MMIC) used for high frequency switching, and more particularly to a high frequency switch device which has high electric power, low insertion loss, high isolation and high switching speed characteristics.
2. Description of the Related Art
Mobile telecommunication equipment such as a mobile phone or wireless LAN generally utilizes microwave in a GHz band, and a switch device that converts high frequency signals by means of a conversion circuit of an antenna or a transmit-receive shifting switch is widely available. Switch devices adopt a High Electron Mobility Transistor (HEMT) that is a compound semiconductor transistor of offering favorable transmission characteristics in a high frequency band, or a Field Effect Transistor (FET) such as a MEtal-Semiconductor FET (MESFET). Especially, GaAs compound semiconductor FETs are frequently used and, accordingly, a MMIC in which a switch circuit itself is integrated in a GaAs substrate is being actively developed.
The high frequency switch circuit requires an insertion loss as small as possible while enhancing isolation, and more particularly, designing of a device serially connected to a signal path is important. A reason of using the GaAs FET as the high frequency switch device is in that GaAs with the higher electron mobility than Silicon (Si) involves small resistance, thereby lowering the loss. Furthermore, GaAs is a semi-insulated substrate, which is thus suitable for achieving high isolation between signal paths.
In order to make a resistance of a channel region of a transistor small as possible in a conventional high frequency switch device, the channel width was designed as wide as possible. By doing so, a surface area of the channel region is secured to thus decrease the insertion loss. However, the widened channel width increases a capacitance caused by a Schottky contact formed between a gate electrode and a channel region. Therefore, an input signal of the high frequency is leaked from the Schottky contact to thereby deteriorate the isolation. Conventionally, the above-stated isolation is improved by installing a shunt FET, which however increases a chip size and, in turn, increases cost.